Since the advent of electronic computers several decades ago and the subsequent improvements in computation speed and efficiency, there has been an increasing need for a practical means to rapidly store and retrieve large volumes of digital data. Many types of mass storage devices have been developed, and after much trial and error what is commonly referred to as the rigid disk drive or "hard drive" has become widely accepted. Nearly every computer presently sold is accompanied by one or more hard drives. Explosive growth in the use of digital communications and multimedia has driven the demand for data storage even higher, while at the same time the acceptable data access latency has shrunk dramatically--computers have no moving parts, and continue to double in speed about every eighteen months. Unfortunately, conventional disk drives are limited by mechanical considerations and thus have a difficult time keeping up with the developments.
In many of the most demanding applications, industry has resigned itself to the considerable expense of Redundant Arrays of Inexpensive Disks, referred to as RAID, often augmented by a large bank of fast-access semiconductor cache RAM, to increase storage capacity, reliability, data transfer rate and access speed. However, increasing the number of disk drives does not decrease the worst-case data access time; a semiconductor cache memory only speeds access to the small fraction of data stored in the memory; and using disk arrays can more than double the cost per unit of data storage. In other applications the need for greater storage capacity and higher performance is also accompanied by the need for high durability and shock resistance, plus minimum size, power consumption, and of course, cost.
An example of a typical hard drive apparatus 10 is represented in FIGS. 1A and 1B. In general, the illustrated configuration includes a motor driven spindle 12 for supporting disks 14 and 15 for rotational motion about the spindle axis. For each disk surface, the apparatus also includes a read and/or write head 13 supported on a head slider 16, which is supported by a load beam 18 and a high speed actuator 20. The actuator 20 is controlled to selectively move the head slider 16 radially across the disk surface, to locate the head adjacent selective data tracks on the disk. The illustrated apparatus also includes read/write and actuator control electronics 22 and voice coil motor means 23, for controlling the operation of each actuator 20 and head 13.
As shown in the side, cutaway view of FIG. 1B, the two disks 14 and 15 are mounted on a single spindle 12 with one slider devoted to each disk surface. Actual drives have employed more or less than two disks. The spindle 12 is rotatably driven, through a conventional high speed bearing 17, by a motor 19.
As shown in the enlarged view in FIG. 1C, each slider 16 is suspended by a flexure 24 from the end of a load beam 18. The mass of the load beam 18 forces the slider 16 and head 13 toward the surface 26 of the disk. As the disk moves (in the direction represented by arrow 28) a film of rapidly moving air 30 is produced by disk motion. The rapidly moving air 30 along the disk surface creates an "air bearing" effect, which inhibits the slider 16 from making physical contact with the disk surface 26, thus minimizing wear.
The head/disk separation distance, also known as slider "flying height", is a critical parameter--decreasing it improves signal strength and, thus, allows higher data recording densities, but decreasing it too much can prompt a head "crash" in which the head impacts the disk surface, damaging or destroying one or both of them. Flying height has been steadily decreased over the years until disk drives are now designed with flying heights of 100 nanometers or less. Efforts are underway to decrease flying height even further, and various methods have also been proposed (see below) to minimize flying height variation caused by differences in disk surface speed relative to the head over the range of head travel.
High performance drives currently available, with disks 90 mm (3.5") in diameter, have a worse-case seek time (the time required to move the read/write head from the data track at one end of its range to the track at the other end) of nearly 20 milliseconds. In addition, they have a maximum rotational delay of 6 milliseconds (spinning at 10,000 RPM's). Thus the time required simply to begin a data transfer can be almost 26 milliseconds--a relatively long period of time for computers, especially those which handle data requests from thousands of simultaneous network users. Note that these times are for some of the highest performance disk drives. More typical drives are significantly slower. Improvements have been made in disk rotation rates, but increases in rpms promote an increase in shaft bearing friction, an increase in friction between disk surfaces and air in the disk chamber, increased power consumption by the disk motor to overcome this friction, increased heat production, and reduced longevity.